Conventionally, a warning device that outputs a sound to notify others of the approach of a vehicle is disclosed in Japanese Patent Laid-Open No. 2005-289175 (JP '175). The vehicle warning apparatus of JP '175 generates a warning sound from a dynamic speaker.
A vehicle warning apparatus may also use a different type of speaker that provides a strong directivity to generate a warning sound, such as a parametric speaker. The parametric speaker is a device that emits, from a supersonic speaker, an audible sound (i.e., a notification sound) after supersonic modulation of the wave form signal of the audible sound. The supersonic sound (i.e., a non-audible sound) emitted from the supersonic speaker has a modulated sound component that is self-demodulated during the transmission of the wave in air, thereby generating an audible sound (i.e., the notification sound) at a distant position from the vehicle.
The supersonic speaker used in the parametric speaker may have snow or other foreign matter attached on a sound emission opening of the supersonic speaker. When foreign matter accumulates on the sound emission opening, the accumulated foreign matter reflects the supersonic sound due to the nature of the supersonic sound. That is, even if the foreign matter is very light object, such as snow, the supersonic sound from multiple supersonic transducers is reflected by the snow on the sound emission opening, decreasing the sound pressure of the supersonic sound before it is emitted through the sound emission opening towards an area outside the vehicle.